[Yandex Cloud documentation](../../index.md) > [Yandex Managed Service for MySQL®](../index.md) > Concepts > Resource relationships

# Resource relationships in Managed Service for MySQL®


The main entity Managed Service for MySQL® operates is a _cluster_.

Each cluster consists of one or multiple _database hosts_, which are virtual machines with deployed DBMS servers. Cluster hosts may reside in different availability zones and even different availability regions. You can learn more about the Yandex Cloud availability zones in [Platform overview](../../overview/concepts/geo-scope.md).

A cluster with two or more hosts is natively [highly available](high-availability.md) because hosts can step in for one another as the cluster’s primary replica. To learn more about how the number of hosts affects cluster availability, see [this section](high-availability.md#host-configuration).

The minimum number of hosts per cluster depends on the selected [disk type](storage.md). The default cluster configuration offered in the management console includes two hosts.

{% note warning %}

We do not recommend creating a cluster having a single host. While being cheaper, such a cluster will not ensure high availability.

{% endnote %}


VMs for cluster hosts can be hosted on:

* _Regular Yandex Cloud hosts_.

    These are physical servers for hosting cluster VMs. They are randomly selected from a pool of available hosts that meet the selected cluster configuration.

* _Dedicated Yandex Cloud hosts_.


    These are physical servers reserved exclusively for your VMs. VMs on dedicated hosts have all features of regular VMs; additionally, they are physically isolated from other users' VMs.

    Dedicated hosts are selected from _dedicated host groups_ specified when creating a cluster.

    For more information, see [Dedicated host](../../compute/concepts/dedicated-host.md).


When creating a cluster, specify:
* _Host class_: VM template for deploying cluster hosts. For a list of available host classes and their specs, see [Host classes](instance-types.md).

* _Environment_: Environment where the cluster will be deployed:
    * `PRODUCTION`: For stable versions of your applications.
    * `PRESTABLE`: For testing purposes. The prestable environment is similar to the production environment and likewise covered by an SLA, but it is the first to get new features, improvements, and bug fixes. In the prestable environment, you can test new versions for compatibility with your application.


You can access a cluster created in a folder from any VM in the same cloud network. For more information about networking, see [this Virtual Private Cloud guide](../../vpc/index.md).


{% note alert %}

The service can automatically access the system and diagnostic information of your databases to render monitoring charts and maintain your cluster health. It never accesses or uses the data you entered into the database manually.

{% endnote %}

Be mindful of what is managed by the service and what, by the Yandex Cloud customer. Understanding these zones of control makes you an efficient user of cloud resources and helps avoid potential database-related issues. For more information, see [Zones of control between managed database (MDB) service users and Yandex Cloud](../../overview/concepts/mdb-responsibilities.md).